An official complaint alleging that British intelligence officers colluded in the torture of a British medical student who was detained in Pakistan after the July 2005 suicide attacks in London has been lodged with the tribunal that conducts investigations into MI5 and MI6.

Labour backbencher John McDonnell has complained to the Investigatory Powers Tribunal (IPT) that the student, his constituent, was picked up by a Pakistani intelligence agency and tortured for two months in a building opposite the British deputy high commission in Karachi.

The student told McDonnell on his release that he was questioned by British intelligence officers, who he believes were from the Security Service, MI5, after he had been tortured. McDonnell believes that British officials "outsourced" his mistreatment to the Pakistani agency, and wants the IPT to examine the matter.

Earlier this year four British nationals claimed they were mistreated after being detained by Pakistani intelligence agents, and that they were then questioned by British intelligence officers in between or after torture sessions.

One has since been convicted of terrorism offences after being returned to the UK, a second is awaiting trial, and a third absconded while subjected to a control order.

Yesterday the Guardian reported that three other Britons - including McDonnell's constituent - have also alleged they were mistreated after being detained in Pakistan, and were eventually released without charge.

MI5 asked the Home Office to issue a statement which said: "The Security and Intelligence Agencies do not participate in, solicit, encourage or condone the use of torture or inhumane or degrading treatment. For reasons both ethical and legal, their policy is not to carry out any action which they know would result in torture or inhuman or degrading treatment."

It is unclear how many Britons have been held in Pakistan for questioning during counterterrorism investigations in recent years.

Earlier this year the Foreign Office responded to a parliamentary question from Andrew Tyrie, Tory MP and chairman of the all-party parliamentary group on extraordinary rendition, by saying there were six such cases since 2000. But the Guardian is aware that there have been at least 11, and there are unconfirmed reports that there may have been more.

Some of the detainees received no assistance from British consular officials. The Foreign Office maintains that it had no duty to represent them while they were in Pakistan as they have dual nationality. The men's lawyers said this claim is undermined by the strenuous efforts that British diplomats make on behalf of the 200-plus dual nationals forced into marriage in Pakistan each year.

When Tyrie asked about MI5's ability to visit and question detainees to whom consular officials claim to have been denied access, Kim Howells, the Foreign Office minister, replied: "Priority was given to the welfare of the detainees."

A number of the detainees themselves deny this, saying that the British intelligence officers who interviewed them appeared to ignore their complaints that they were being tortured.